A frequency synthesizer may generally be described as an electronic system for generating any of a range of frequencies from a reference signal having a particular frequency. Frequency synthesizers may be found in many different kinds of devices, such as radio transceivers, mobile telephones, mobile radio base stations, satellite transceivers, GPS systems, etc.
In for example a mobile radio application, a signal to be used when transmitting and receiving data is conventionally generated by a frequency synthesizer comprising a phase-locked loop (PLL). A PLL frequency synthesizer may for example have a reference signal with a certain frequency as input, and a programmable counter which may generate a comparison signal by dividing an output signal outputted from the PLL frequency synthesizer. The output signal from the PLL frequency synthesizer may be an output signal outputted from an oscillator also comprised in the PLL frequency synthesizer. The PLL frequency synthesizer may further comprise a phase comparator for comparing the phase of the input reference signal with the phase of the comparison signal from the programmable counter and produce an output signal in dependence of this phase difference. From this output signal, a control signal (e.g. a tuning voltage) may be generated and used as input to the oscillator in order for the oscillator to generate and output an output signal having a specific frequency. This is the output signal outputted from the PLL synthesizer.
The PLL and the oscillator comprised in the PLL frequency synthesizer may be designed for different type of applications, some which have high requirements on the noise performance of the output signal and other applications with relative low requirements on the noise performance of the output signal. Examples of applications with high requirements on the noise performance of the output signal are network units or nodes, such as, for example, mobile radio base stations. Examples of applications with low requirements on the noise performance of the output signal are terminal units, such as, for example, mobile telephone applications. However, many oscillators implemented today result in signal-to-noise ratios on the output signal outputted from the PLL frequency synthesizer that does not fulfill the increasingly higher requirements on the noise performance of many existing and new emerging applications on the market. Thus, there is a demand for oscillators with improved noise performance.
Also, the PLL in a high performance PLL frequency synthesizer is conventionally designed as an integrated circuit (IC), while the oscillator in the PLL frequency synthesizer is designed using discrete electrical components, such as, e.g. transistors, capacitors, inductors, etc. Using different technologies such as an integrated PLL circuit and a discrete component oscillator to form a PLL frequency synthesizer does not enable a compact solution from a miniaturization perspective, as well as it does not provide a particularly cost effective solution from a manufacturing standpoint. These are also important factors in contemporary design of various telecommunication and radio network applications.